The present invention relates to bone block implants which promote bone fusion between adjacent vertebrae.
Intervertebral spinal inserts are used to provide support and maintain normal distance between adjacent vertebrae in cases where a patient""s vertebral discs have degenerated. Such degeneration can occur as a result of aging or trauma and typically results in pinched or damaged nerves between or proximal to the adjacent vertebrae. Moreover, such discal degeneration causes shifting of the loading along the patient""s spinal column, which in turn further accelerates the vertebral degeneration.
Intervertebral inserts are typically used to reestablish normal intervertebral spacing and to cause fusion between adjacent vertebral bodies.
A common problem with the existing intervertebral spinal inserts is that they do not provide stabilization in two perpendicular directions in the plane of the patient""s intervertebral space.
Another disadvantage is that, during such major surgery, the actual insertion of the intervertebral insert requires distraction of the adjacent vertebrae to first open a sufficiently large passage for the insertion of the insert therebetween. Such distraction is typically performed by dedicated instrumentation and invasive tools which must first enter the intervertebral space and then grip and hold apart the adjacent vertebrae.
The present invention provides methods and devices for inserting a bone block into a patient""s intervertebral space wherein the bone block is supported in an inserter which is first advanced into the intervertebral space. After the inserter is received into the patient""s intervertebral space, the inserter is then rotated by approximately 90xc2x0. Convexly curved outer camming surfaces on the inserter operate to separate the adjacent vertebrae as the inserter is rotated by 90xc2x0. Rotation of the inserter by 90xc2x0 also operates to orient the bone block in a preferred orientation relative to the opposite vertebral surfaces. An illustration of camming apart adjacent vertebrae using an outwardly facing convexly curved camming surfaces is set forth in provisional patent applications Serial Nos. 60/086,945 filed May 27, 1998; 60/113,651 filed Dec. 23, 1998; and 60/120,663 filed Feb. 19, 1999; incorporated herein by reference in their entirety.
Specifically, the outwardly facing convexly curved camming surfaces are adapted to engage, and to separate by camming action, the opposed adjacent vertebrae when the bone block is initially placed between the vertebrae and then subsequently rotated by 90xc2x0. After the bone block is rotated into position, it supports the spinal load, thereby easing pressure on the vertebral disc and surrounding tissue. As such, prior distraction of the adjacent vertebrae with dedicated instrumentation is either not required, or is substantially minimized.
After the bone block is rotated into an anchored position between the adjacent vertebrae, the inserter is withdrawn from the intervertebral space leaving the bone block in a preferred position to promote bone fusion between the adjacent vertebrae. In a preferred aspect, the bone block is held stationery by a push rod, (which is preferably received in an inner cannulated passageway in the inserter), and the inserter, (preferably positioned thereover), is withdrawn such that the bone block is pushed out of the distal end of the inserter.
In preferred aspects, the bone block is dimensioned to extend to a height greater than that of the inserter such that vertebral supporting surfaces of the bone block anchor against the adjacent vertebrae to facilitate removal of the bone block from the inserter, reducing or eliminating the requirement of a push rod separating the bone block from the inserter.
The present bone blocks can be used singly, in pairs, or in quartets. When used in pairs or quartets, the bone blocks can be angled with respect to one another such that increased vertebral stability is achieved. Similarly, more than four bone blocks can be uses, and the present invention therefore also encompasses using 6, 8, 10 or more bone blocks to provide intervertebral stability.
In a first preferred aspect of the invention, the inserter is received through a cannula which is percutaneously introduced into the patient in a posterolateral approach. Also in preferred aspects of the invention, the cannula has an oval or racetrack shaped cross-section and the inserter received therein has a truncated oval shaped cross-section.
In an alternate aspect of the invention, a separate cannula with an inserter received therethrough is instead replaced by a single unit, being an oval shaped cannula which is dimensioned to support a bone block at its distal end. In this aspect of the present invention, the outwardly facing camming surfaces which operate to cam apart the adjacent vertebrae are disposed on the distal end of the cannula itself and the vertebrae are cammed apart as the cannula is rotated by 90xc2x0.
An important advantage of the present invention is that it provides a system for implanting bone blocks in a patient""s intervertebral space in a minimally invasive surgical procedure. In contrast, current interbody fusion devices are typically implanted during open surgery.
An advantage of approaching posteriolaterally in a minimally invasive procedure is that the passive elements of spinal stability (anterior and posterior longitudinal ligaments, interspinous ligaments, and facet capsule) are not disturbed and provide stability when stretched by the insertion of the bone block.
The present invention also provides methods for positioning first and second bone blocks in the patients intervertebral space. Preferably, the first and second bone blocks are disposed with their central longitudinally extending axes at an angle to one another so as to give increased vertebral stability. In this aspect of the invention, each of the bone blocks are preferably introduced through percutaneous cannula which are oriented in opposite posterolateral approaches, being disposed at about 70xc2x0 to 135xc2x0, and most preferably 90xc2x0, to one another.
In various aspects of the invention, the first and second bone blocks may optionally be interlocked together in the patient""s intervertebral space. In such aspects of the invention, the first and second bone blocks may be interlocked by a variety of techniques including suturing the blocks together, interlocking a protrusion on the first bone block with an aperture on the second bone block or by interlocking a notch on the first bone block with a groove on the second bone block. In addition, the first and second bone blocks may be fastened together by a fastening pin.
The present invention also provides a system for introducing a bone block into an intervertebral space comprising a two pronged inserter wherein each prong has an outwardly facing convexly curved camming surface for separating adjacent vertebrae and wherein each prong is disposed on opposite sides of the bone block positioned therebetween. The bone block may preferably have lateral protrusions which extend in a longitudinal direction along the length of the bone block. In this aspect, the lateral protrusions on the bone block preferably mate with longitudinally extending grooves on the inner surfaces of the prongs, thereby preventing unwanted motion of the bone block, but permitting the bone block to be slid axially out of the distal end of the inserter.
An advantage of the present system is that the bone block inserter protects the bone block during insertion, and distracts the adjacent vertebrae by camming action to gain the needed space for placement of the bone block. By distracting the adjacent vertebral bodies, the present invention also decompresses the nerves that may be causing pain.
In another aspect of the present invention, a bone block is provided having opposite vertebral contact surfaces with opposite sides spanning between the vertebral contact surfaces, wherein the opposite vertebral contact surfaces each have a width which is about 20% to 60%, and most preferably 30% of the height of the opposite sides spanning between the opposite vertebral contact surfaces. In this aspect of the invention, a tall, narrow bone block is provided for positioning between adjacent vertebrae.
In preferred aspects, the opposite vertebral support surfaces of the bone block can be angled with respect to one another to restore a patient""s lordotic angle.
An advantage of the present bone block relative to existing cortical bone blocks is its novel shape which uses substantially less human tissue. Specifically, the present bone block uses ⅓ to ⅕ the amount of human tissue currently used in existing bone block implants. The advantage of using less human tissue is important in the business of bone banks as the supply of donor tissue is quite limited. The present bone block may preferably comprise any suitable bone material including autologous, allographic, xenographic, or other osteoinductive and osteoproliferative elements.
Another advantage of the present system is that the bone block is placed between the vertebral endplates to rest upon cortical bone. In contrast, current fusion cages and cortical bone blocks require predrilling and partial destruction of the vertebral endplates. This predrilling removes cortical bone from the endplate thereby increasing the likelihood of subsidence or the sinking of the bone block into the vertebral body.